1. Field of the Invention
This invention relates to an image read-out apparatus wherein a recording medium, such as X-ray film or a stimulable phosphor sheet, on which an image has been recorded, is scanned in a main scanning direction and a sub-scanning direction, and an image signal representing the image is thereby obtained.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In the radiation image recording and reproducing systems described above, different sampling densities should be employed during the detection of image signals representing different types of images. Usually, the type of an image is designated, and an image signal is detected from the image with a sampling density suitable for the designated type of the image. By way of example, images are classified into types in accordance with what mode was used when the image was recorded (e.g., an ordinary image recording mode, an enlarged image recording mode, or tomography) and what portion of an object is represented by the recorded image (e.g., the head, the neck, the chest, or the abdomen in cases where the object is a human body).
However, even for images of the same type (e.g. a plurality of images which represent the same portion of an object and which are recorded in the same mode), the necessary sampling density varies in accordance with from what point of view a person aims at observing a visible image reproduced from the image signal. If a sampling density different from the necessary sampling density is employed, the problem often occurs in that the details of an image, which should be found, cannot be found. For example, an X-ray image of the chest of a human body is usually used during diagnosis of a tuberculosis or a cancer of the lungs. Therefore, when the type "ordinary image recording mode, frontal chest" is selected during the image read-out operation, an image signal is obtained with a sampling density suitable for making a diagnosis of an ordinary disease, such as a tuberculosis or a cancer of the lungs. However, it often occurs that an interstitial disease occurring between alveoli of the lung should be diagnosed. In such cases, the problem occurs in that details cannot sufficiently be found with a visible image, which is reproduced from an image signal detected with the ordinary sampling density. As another example, when the type "ordinary image recording mode, frontal kneepan" is selected during the image read-out operation, an image signal can be obtained with a sampling density which is suitable for making a diagnosis of an ordinary broken bone. However, in cases where a disease of a bone caused by lack of calcium is to be diagnosed, the problem occurs in that a diagnosis cannot be made accurately with a visible image, which is reproduced from the image signal obtained with the sampling density suitable for making a diagnosis of an ordinary broken bone.
It may be considered that, in order to cope with special diagnoses described above, every image signal be detected with a high sampling density. However, with such a technique, the amounts of image signals become very large, and therefore storage devices having a very large storage capacity must be used. Thus such a technique is not suitable from the point of view of the image filing efficiency.